A liquid crystal display comprises pixel units designed in a matrix form, and a driving circuit for driving these pixel units. The deflection of liquid crystal molecules is achieved by means of change in the electric field in the liquid crystal cell, thereby achieving the display effect.
TFT-LCD is an active semiconductor device. Its display principle is to convert incident light emitted from the backlight into linearly polarized light by means of a first polarizer, which polarized light passes through a liquid crystal cell formed by attaching two layers of glass and injecting liquid crystal therebetween, and is emitted via a second polarizer. The liquid crystal has an optical birefringence property and would generate optical retardation for different incident light or different vibration directions, thereby changing the polarization state of the incident light. Since the liquid crystal molecules are arranged in different manners under different electric field intensities, the amounts of optical retardation generated are different, the polarization states of the incident light arriving at the second polarizer are different, and the intensities of the exit light are also different, such that the brightness of the exit light is adjusted by different liquid crystal electric field intensities and display of different gray scales is formed.
Typically, a pillar-shaped spacer (PS) is provided between the array substrate and the color film substrate in a liquid crystal display panel. The current design of PS is classified into PS on TFT and PS on Gate in terms of the position of PS, wherein the PS on Gate is further divided into PS on Gate with pillow-shaped spacer and PS on Gate without pillow-shaped spacer; and is generally classified into main PS and sub PS in terms of the height of PS, wherein the height of the main PS is relatively high and plays the role of static support in cell alignment of the array substrate and the color film substrate to ensure that the cell gap between the array substrate and the color film substrate reaches a design value, and the height of the sub PS is lower than that of the main PS and used for maintaining the cell gap when the liquid crystal cell is subjected to a vertical pressure and the main PS is heavily compressed.
The brightness of the dark state (L0 gray scale) is an important indicator of a TFT-LCD product. If the brightness of the dark state is high, it affects the contrast of the product, and in severe cases the picture cannot be normally viewed. Thus, to reduce light leakage in the dark state is a major issue of design of the TFT-LCD product.
In addition to the layout design and circuit design of the TFT-LCD, a further important cause for light leakage in the dark state is additional optical retardation resulting from deformation of glass when the liquid crystal screen is subjected to an external force (which may be an interference caused by design mismatch or imprecise manufacture of the mechanism, forming a local external force; and may be an external force applied on the liquid crystal screen), which changes the polarization state of the incident light passing through the liquid crystal cell in a normal state, thereby causing light leakage. In particular, for a notebook computer and a TPC product, this undesirable phenomenon becomes more common and obvious since they often need to be locally applied with an external force in use (for example, during the opening and closing process of the notebook computer).
According to the theory of photoelasticity, even if glass is an isotropic material, an anisotropic phenomenon would also take place therein under the effect of external force, that is, the refractive indexes under different stress states are not the same. The refractive index of a photoelastic material at a certain point is directly related to the stress state of this point. For a liquid crystal display, the stress is mainly caused by the frictional force between the PS and the array substrate, the magnitude of which is directly proportional to the friction coefficients and the contact area of the PS and the array substrate. The increase in stress results in birefringence of glass and the resulting optical retardation is directly proportional to the magnitude of stress and the thickness of glass.
In the prior art, in order to reduce the light leakage caused by the additional optical retardation of glass, an effective improvement method is to reduce the thickness of glass. However, too thin glass cannot be used directly on the high-generation TFT-LCD production line. Using thinning technology would further cause problems including complex process, rising cost and pressure in environmental protection.
In addition, since the designed density of the main PSs is relatively low, when they are subjected to an external force, a great internal stress is generated at local points inside the glass, resulting in obvious light leakage. In contrast, since the designed density of the sub PSs is very high and present in almost every pixel, when an external force is applied, the substrate bears the force uniformly and disperses the stress, not causing light leakage and whitening. Experiments prove that the light leakage and whitening are mainly caused by an internal stress of glass resulting from the friction between the main PSs and the array substrate under the effect of external force. When a TFT-LCD product is subjected to an external force, the glass is deformed to generate an internal stress and thereby produce additional optical retardation, resulting in light leakage in the dark state. In severe cases, the whole screen is whitened so that the picture cannot be normally viewed.
In conclusion, in the prior art, the friction between the pillar-shaped spacer and the array substrate of the display panel is increased under the effect of internal stress, thereby the amount of additional optical retardation is increased, resulting in the phenomenon of light leakage in the display panel in the dark state.